1. Field of the Invention
The present invention generally relates to a mounting structure for a semiconductor element, and particularly relates to a mounting structure for a semiconductor element wherein a semiconductor bare chip is bonded through an adhesive film to a die pad on a board, and wherein a wire is bonded to and extends between a pad on the upper surface of the semiconductor bare chip and a pad near the die pad.
2. Description of the Related Art
Electronic devices for processing signals have electronic circuit modules therein. Some connectors for high-speed signal transmission also have electronic circuit modules therein. In such an electronic module, for example, a semiconductor bare chip is bonded to a die pad of a printed circuit board, and a wire is bonded to and extends between a pad on an upper surface of the semiconductor bare chip and a pad near the die pad.
Along with improvement in signal processing speed, the length of the wire extending between the pad on the upper surface of the semiconductor bare chip and the pad near the die pad has become recognized as a cause for lowering signal transmission accuracy.
A related-art semiconductor bare chip mounting structure 10 is shown in FIGS. 1A and 1B, in which a portion encapsulated by resin is not shown for clarity. In the semiconductor bare chip mounting structure 10, a semiconductor bare chip 20 is mounted on the upper surface of a printed circuit board 11 through application of Ag paste, and wires 30 extend between the semiconductor bare chip 20 and the vicinity thereof. The reference numeral 40 indicates a cured Ag paste film. The Ag paste is conductive adhesive, and is a mixture of a paste-type thermosetting adhesive with Ag particles.
FIG. 2A shows a part of the printed circuit board 11. A square-shaped die pad 13 and terminal pads 14 are formed on the upper surface of a board main body 12. The terminal pads 14 are arranged around the die pad 13. The area surrounded by a double-dot chain line is a semiconductor bare chip mount area 15 on which the semiconductor bare chip 20 is to be mounted. Each terminal pad 14 is spaced apart from the semiconductor bare chip mount area 15 by a distance A. The distance A is in a range from about 0.5 to 0.8 mm because a portion 40a of the Ag paste film 40 extending outside the semiconductor bare chip 20 might be relatively large.
The semiconductor bare chip 20 is a square-shaped piece of a wafer, and pads 21 are arranged along the outer circumference of the upper surface thereof.
The wires 30 are made of Au, each extending between the semiconductor bare chip 20 and the corresponding terminal pad 14 with one end bonded to one of the pads 21 and the other end bonded to the corresponding terminal pad 14.
Referring to FIGS. 2A and 2B, a predetermined amount of the Ag paste is applied onto the center of the die pad 13 by a potting machine (not shown) so as to form a generally ball-like Ag paste mass 41. Referring then to FIGS. 3A and 3B and FIGS. 4A and 4B, the semiconductor bare chip 20 is placed onto the Ag paste mass 41. The Ag paste mass 41 is pressed by the semiconductor bare chip 20, so that the Ag paste spreads on the upper surface of the die pad 13 as indicated by arrows 50 of FIG. 3A so as to be an Ag paste film 42 and then be an Ag paste film 43. The Ag paste film 43 is cured by heating, so that the semiconductor bare chip 20 is bonded onto the die pad 13 through the cured Ag paste film 40. The cured Ag paste film 40 extends across the entire lower surface and beyond the peripheral edge of the semiconductor bare chip 20.
<Patent Document 1> Japanese Patent Laid-Open Publication No. 1-297828
As the upper surface of the die pad is flat and smooth, the Ag paste applied on the upper surface of the die pad has high surface tension. Therefore, when pressed into a film, the Ag paste does not smoothly spread out and, in some directions, might not be large enough in amount.
In view of that, a volume V1 of the Ag paste mass 41 is greater than a logically required volume V0. Thus, the Ag paste film 43 is formed to extend beyond the peripheral edge of the semiconductor bare chip 20. The presence of this portion extending beyond the peripheral edge of the semiconductor bare chip 20 ensures bonding between the entire lower surface of the semiconductor bare chip 20 and the die pad 13.
The portion of the Ag paste film 43 extending beyond the peripheral edge of the semiconductor bare chip 20 is indicated by the reference numeral 40a in FIGS. 4A and 4B.
In the related art, as described above, when the Ag paste is pressed into a film, the Ag paste does not smoothly spread and, in some direction, might not be large enough in amount. Therefore, the volume V1 of the Ag paste of the Ag paste mass 41 is made greater than the logically required volume V0 by an additional volume V10 just to be safe. This makes a greatest width W1 of the portion 40a extending beyond the edges of the semiconductor bare chip 20 relatively large, which is in a range from about 0.2 to 0.5 mm.
If the surfaces of the terminal pads 14 are covered with the Ag paste, the wires 30 cannot be bonded to the terminal pads 14. To prevent this problem, the terminal pads 14 need to be formed at positions where the portion 40a does not reach. In FIG. 1B, the terminal pads 14 are spaced apart from the peripheral edge of the mounted semiconductor bare chip 20 by the distance A, which is in a range from about 0.5 to 0.8 mm. A length L1 of each wire 30 is about 1.2 mm. If, in the future, signal transmission becomes faster than now, reduction of signal transmission quality in the wires 30 might be a problem.
It is to be noted that a pressure membrane printing machine may be used in place of the potting machine. A pressure membrane printing machine is able to print an Ag paste film only on an area of the upper surface of the die pad 13 where the semiconductor bare chip 20 to be mounted. Therefore, when the semiconductor bare chip 20 is mounted, almost all of the Ag paste film is covered with the semiconductor bare chip 20. Accordingly, the terminal pads 14 may be located closer to the peripheral edge of the mounted semiconductor bare chip 20, which allows reduction of the length of the wires 30. However, the pressure membrane printing machine requires dedicated tools such as a printing screen, resulting in increase in the cost of the electronic circuit module. Moreover, if the semiconductor bare chip 20 is modified, a new printing screen needs to be prepared, which leads to a slow response to design change.
Using the potting machine for applying Ag paste is advantageous especially when the number of semiconductor bare chips to be mounted is small. In the case of electronic circuit modules embedded in connectors, because only a few semiconductor bare chips are mounted, the potting machine is used.